Page 24, O2 Uptake, Exercise & Cor. Circ., Dr. D. Penney

Coronary Circulation (continued..):

A condition of inadequate oxygen delivery can occur in two ways:

  • reduced oxygen carriage by the blood in the presence of normal flow, due to inadequate availability of oxygen at the lung (e.g. high altitude), impairment of hemoglobin's ability to carry oxygen (e.g. carbon monoxide hypoxia), or inadequate hemoglobin (anemia), or
  • because of reduced blood flow. The first condition is referred to as hypoxia, while the second and more common condition is known as ischemia.

    Specifically, ischemia is a condition of oxygen deprivation accompanied by the inadequate supply/removal of other metabolites consequent to reduced perfusion. An abnormally increased A-V oxygen difference at rest usually indicates ischemia. The heart's dilemma is that it operates with a high A-V oxygen difference but cannot extract all of the oxygen from blood passing through it. Ischemia may be relieved by increasing blood flow, as with vasodilators, or by decreasing oxygen consumption. For the heart, the latter can be accomplished through the use of beta-blockers, Ca++ channel blockers, diuretics, etc.

    During exercise in a healthy heart, coronary blood flow can increase 3-4 fold as the result of coronary vasodilation (Figure 3.09).

    The heart normally uses lactate as a fraction of its fuel, but in ischemia or hypoxia, lactate is produced. Thus increased A-V lactate also indicates inadequate blood flow.